Broad band amplifier



Oct. 27, 1942.. H, NYQUIST BROAD BAND AMPLIFIER Filed Sept. 20, l940 AMA AAAA

VII

HI I I Y lNVE/VTOR By HJVkQU/ST ATTmA/Ey t Patented Oct. 27, 1942 UNITED STATES PATENT OFFICE BROAD BAND AMPLIFIER Harry Nyquist, Millburn, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September 20, 1940, Serial No. 357,517

3 Claims.

same as R1. A feedback connection extends from an intermediate point on R! to an intermediate point on R0, this connection including a series resistance Rs. While for convenience there are shown several batteries B1 and B: it is understood that there is but one battery 131 common to all stages of the amplifier and that there is -'but one batteryBz common to the grid circuit such an amplifier in which the various stages may be supplied from common batteries or in which a plurality of such multistage amplifiers may be.

supplied from common batteries.

In connection with the amplification of television signals difficult problems arise because of the wide band of frequencies transmitted, giving a ratio of the order of 100,000 between the highest and the lowest frequencies. The provision of a band width of this ratioleads to difficulties from the standpoint of transfer design and from the standpoint of delay distortion, particularly when feedback is desired. I find that many of these problems disappear or are greatly diminamplifier shown a'feedback amplifier inserted between two" lines L1 and Li, each with ground return. It will be observed that exclusive of heating batteries the amplifier employs a total of two batteries whichare grounded. The amplifier itself is shown as a three stage amplifier comprising the tubes 4, 5 and'B. The two interstage couplings comprise a resistance R1 in series with the plate battery supply B1, 2. grid resistance R: in

series with the battery B: and a resistance R:

connected to the resistance R1, R2 and from the plate of the tube to the grid of the succeeding tube. The input of the first tube comprises the resistance R0 connected to ground through B2. The output of the last tube comprises the resistance R4 also connected to ground through B2 of the various stages,

Let it'be assumed that the positive battery supplying current to the plate is 200 volts. The proper plate and grid potentials for the particu lar tubes used and for the desired points of economically. There will be some loss of output operation are obtained by adjusting the various resistances shown. The points where the feedback resistance Re is connected are preferably so chosen that there is no current flowing through the feedback unless current is received from the line. This, however, is not an important requirement. Such a circuit as thus described will normally function as an amplifierwhich is independent of the frequency of the impressed signal. In some cases there may be a tendency for the circuit to sing at high frequencies due to the interelectrode and other capacitances present in the circuit. such tendency towards singing can be prevented by at a. i

A readily observed advantage of the circuit as described is its simplicity; the amplifier consisting of nothing but tubes and resistance elements. There will obviously be a constant drain of battery powerthrough the resistance networks, involving both an increase in the voltage and the amperage of the battery over that used in conventlonal amplifiers. On the other. hand it is possible to select the tubes and resistances so that the waste power is not a serious factor powerin the bridged resistance but this may be remedied in a large degree by inserting an inductance 8 at the point D of Fig. 1 as shown in Fig. 2. This will cause high frequencies to be transmitted somewhat better than low, which would usually be an advantage.

Instead of using the batteries and B: asshown, one may use a generator or a rectifier be a function of frequency, thus giving rise to a variation of gain with frequency. To overcome this and incidentally to reduce the effect of noise in the power supply and the effect of interand a resistance R5 which may or may not be the stage coupling through the power supply, a

a condenser connected as shown balanced push-pull amplifier may be used as shown in Fig.3.

with a circuit such as shown in Fig. i it is B batteryin series with the plate of the one tube and the grid of the-next and to ground a suitable point in the battery. This, however, ere

tions may be improved only by the choice of low impedance amplifiers and by the choice of relatively high impedancelines, but this will'frequently not be necessary. As far as the alternating current components are concerned the situation may be improved by the use of net works including transformers. One arrangement for this is shown in Fig. 4. The/transformer here is outside the feedback circuit. It results, however, in transmission differentials as between alternating current and direct current,fbeing in efiect an equalizer.

It will be observed. that there is a loss in the signal wave both because the output impedance stages causes a drop in the signal voltage available at the grid of the next tube. The former of these eifects is not serious in wide band amplicludes the possibility of using common battery and of using chaiging apparatus with high admittance to ground. Moreover, the admittance to ground of the battery is itself high.

An arrangement by which the losses due to resistances R3 may be obviated. is shown in Fig. .5. In this figure the battery supplying the power is as shown in Fig. 1 but-the series interstage reof a tube circuit is lowered by the impedance 7 in series with the positive battery and because i the series resistance of the coupling between the flers because in the usual broad band amplifier it is necessary to lower this impedance artificially to get maximum band width. The second eiiect can be, minimized by suitable design of the circuit constants. pedance has value in broad band transmission because it tends to separate the interelectrode capacities in one tube from those in theother.

In the usual electrostatically controlled tubes the controlled and thecontrollingcircuit have a point in common, namely the cathode. Moreover, the plate and grid differ in potential in the tubes commonly used. If in the usual circuit the plate of one tube is .connected directly to the Moreover, the series imgrid of the succeeding tube, it is required'that the successive cathodes should differ in potential by an amount about equal to the normal potential difierence between the plate and the grid.

This, from the point of view of my invention, is

undesirable be cause it does not permit the successive stages to use common batteries which is an important feature of my invention, nor does it permit the use of charging devices which are grounded orwhich have a high admittance to ground. If it is assumed that the successive tubes should have filaments which are at the same or nearly the saine potential it would be necessary that some element be inserted between the plate of one tube and the grid of the next which insures that there should be a potential difference between them. In the arrangement described in Figs. 1 and 3 the resistance element Rs carrying direct current has been shown and makes it possible to obtain the necessary potential difference between the grid of one tube and its cathode. Thishas the advantage from the standpoint of wide band transmission that a resistance element can be constructed whose admittance to ground is exceedingly small. It

might be argued that the coupling resistances R3 should be replaced by condensers. This, however,is ruled out in direct current amplifiersv because it does not pass direct current. Moreover,

it is objectionable in feedback amplifiers on account of the phase shift present at low frequencies. In addition the admittance to ground tends to be greater-ma condenser than in a resistance.

An arrangement which has been used frebetween a point of positive potential on a comquently in directcurrent amplifiers-is to put the sistances are replaced by a string of counterelectromotive cells l. These cells would be made or such materials and of such number that a small trickle charging current from the batteries is necessary to maintain them and the voltage selves would require little or nomaintenancev It important that the string of cells shall have low electrostatic capacity to ground and this may be obtained by making the plates of the cell quite small. Each cell may be made of thin bimetallic cups about one-quarter inch in diameter andfitting inside one another with suitable separators and electrolyte. Obviously a wide range in choice of metals and electrolytes is available. I

llnFlgafi' I have shown a communication system such as a coaxial line with a plurality of amplifiers of the type described in connection with Fig. 1. These amplifiers A, B and C are connected in tandem with such spacing as would be considered normal for the coaxial line. it

will be observed that the coupling from the final plate in amplifier A to the initial grid in amplifier Bis entirely similar to the interstage couplings Within one of the amplifiers, the coaxialelement being equivalent to the series resistance Ra. So far as direct current effects are conearned the couplings will be the same in form but may difier in magnitude inthat the coaxial link will have the attenuation characteristic of alternating current signalsand in.that the resistance of the coaxial link may be different fromthat of the similar coupling resistances-Ra. In general the system would be operated on such a basis that the attenuation in' a' coaxial link will be equal to the gain in one of the amplifiers,

conductive feedback path from the output to the input of said amplifier forming with said amplifier a negative feedback loop whose voltage transfer factor for a wide frequency range extending to zero frequency is substantially uniform and ofgreater order of magnitude than.

unity; each of the interstage coupling circuits of the amplifier comprising a resistance connected noon source of space current forthe tubes and the plate of one tube, a resistance connected be tween the grid of the following tube and. a point over the whole of negative potential on a source of grid biasing voltage for the latter tube, and a battery of counter-'electromotive force cells connected between said plate and said grid and in series with said resistances and said sources, said battery comprising a series of thin bimetallic cups having a diameter as small as approximately one-quarter .multistage electric space discharge tube amplifier for amplifying substantially Without distortion voltages of a Wide range of frequencies including zero frequency, said amplifier having a conductive feedback path from the output of the amplifier to the input thereof forming therewith a negative feedback loop whose loop gain is substantially constant over said frequency range, each of the interstage coupling circuits of the amplifier comprising a resistance connected between a point of positive potential on a common source of space current for the tubes and the plate of one tube, a resistance connected be tween the grid of the following tube and a point of negative potential on a source of grid biasing voltage for the latter tube and conductive means for maintaining said grid at a lower unidirectional potential than said plate conductively connected between said plateand said grid and in series with said resistances and said space current and grid biasing sources and having its capacity to ground at least as small as approximately ten micro-microfarads, said system comprising also an incoming circuit conductively connected to the-grid of the first stage of said amplifier for supplying thereto unidirectional and alternating voltages tobe amplified thereby, and a utilization circuit conductively connected to the plate of the last stage of said amplifier for receiving therefrom and utilizing amplified unidirectional and alternating voltages.

3. A transmission system comprising a source of television signals of a band of frequencies from zero up to a frequency of the order of a million cycles per second, a plurality of spaced broad band amplifiers adapted to transmit and amplify said signals, one of said amplifiers having its input grid conductively connected to said source, each amplifier being a multistage amplifier having a conductive feedback path from its output to its input forming with the amplifier a nega tive feedback loop whose loop gain is substantially constant over said frequency band, each of the interstage coupling circuits of each ainplifier comprising a resistance connected between a pointer positive potential on a common source of space current for the tubes of the amplifier and the plate of one of said tubes, a resistance connected between the grid of the following tube and a point of negative potential on a source of grid biasing potential for the latter tube, and

conductive means for maintaining the latter grid at a lower unidirectional potential than said plate conductively connected between said plate and the latter grid and in series with said resistances and said space current and grid biasing sources and having its capacity to ground at 

